Satellite communication transmission control system and small aperture terminal

ABSTRACT

A satellite communication transmission control system includes one central station, a plurality of small aperture terminals, and one communication satellite serving as a repeater for the central and aperture terminals. This system performs communication via a frequency division multiple access communication channel. The central station includes a modulator for transmitting via a common channel a signal for controlling/monitoring the small aperture terminals on the basis of a stable clock signal. Each aperture terminal includes a modulator, transmission frequency converter, antenna/RF device, reception frequency converter, demodulator, and frequency comparator. The modulator modulates communication information. The transmission frequency converter frequency-converts a modulated wave from the modulator into a carrier signal (modulated carrier). The reception frequency converter frequency-converts the modulated carrier. The demodulator demodulates a signal received via the common channel and outputs a reception clock. The frequency comparator detects the frequency error of a local oscillator, and generates a control signal for stopping transmission of the modulated carrier until the frequency error falls within a preset tolerance range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital satellite communicationsystem and, more particularly, to a satellite communication transmissioncontrol system which uses one central station and a plurality of smallaperture terminals and performs frequency division multiple accesscommunication via a satellite repeater, and a small aperture terminal.

2. Description of the Prior Art

There is known a satellite communication method for performing frequencydivision multiple access (to be referred to as FDMA hereinafter)communication of exchanging modulated carriers between one centralstation and a plurality of aperture terminals or between the pluralityof aperture terminals via a satellite repeater in a digital satellitecommunication system constituted by the central station and plurality ofaperture terminals.

In the FDMA satellite communication system, the precision of the centerfrequencies of modulated carriers must be set sufficiently high so asnot to influence modulated carriers adjacent along the frequency axis ofa satellite repeater. In other words, the central station and apertureterminals are always inhibited from transmitting unstable signals intransmitting modulated carriers.

When a plurality of modulated carriers A, B, and C are emitted toward asatellite repeater, as shown in FIG. 1A, no problem arises as far as thecenter frequencies of the modulated carriers have high precision. If,however, the center frequency of a given modulated carrier suffers alarge error, this modulated carrier interferes with an adjacentmodulated carrier and is adversely affected by itself, as shown in FIG.1B.

In conventional radio communication, a known transmission control systemin a transmitter is to prevent transmission of a transient unstabletransmission signal upon turning on the transmitter.

For example, Japanese Examined Patent Publication No. 60-056778discloses a radio transmitter which comprises a timer (power-on timer)for turning on only the power supply of a circuit on an antenna sidewith a delay in order to prevent any error caused by partial oscillationduring feedback from the crystal oscillation circuit to the antenna uponturning on the power supply, and which transmits a transmission signalfrom the antenna after a crystal oscillation circuit is turned on.Japanese Patent No. 2944480 discloses a transmission output controlsystem in which a transmitter comprises a controller (CPU), hardcontroller, and power-on reset circuit (power-on timer), a transmissionoutput is controlled by the hard controller upon turning on the powersupply, and after the controller (CPU) is activated, the transmissionoutput is controlled in accordance with reception level information froma partner receiver.

Japanese Unexamined Patent Publication No. 64-001335 discloses atransmitter with a noise generation prevention circuit that detectsnoise generated from a digital phase comparator in the automatic phasecontroller (APC) of the transmitter, and turns on the output-side switchof the transmitter a predetermined time after stopping generation ofnoise, in order not to generate noise on the receiving side even if thevoltage-controlled oscillator (VCO) of the APC having a referencefrequency oscillator is activated from a frequency different from thecenter frequency based on the reference frequency oscillator when thetransmitter is turned on. Japanese Unexamined Patent Publication No.09-135178 discloses a transmission output control system which uses analarm detection circuit for monitoring an output from a phase detector(phase comparator) in a similar APC and controlling ON/OFF switching oftransmission, and a power-on reset circuit (power-on timer), and whichinhibits ON-switching of transmission by the power-on reset circuit(power-on timer) upon turning on the power supply, thereby preventingfluctuations in transmission output caused by unstable operation of thealarm detection circuit.

As described above, each aperture terminal or the like executescommunication by the FDMA method via a satellite repeater in the digitalsatellite communication system for performing communication between onecentral station and a plurality of aperture terminals or between aplurality of aperture terminals via a satellite repeater. Transmissionsignals must be transmitted while their frequency bands are preventedfrom overlapping each other. Particularly, transmission of an unstabletransmission signal must be prevented upon turning on the power supply.

For example, the frequency band of the Ku band often used in a satellitecommunication system in which the aperture terminal is formed from avery small aperture terminal ranges from 14.0 GHz to 14.5 GHz in uplinkto a satellite. The transmission frequency error of the modulatedcarrier is 1 ppm (=1×10⁻⁶). This error corresponds to 14 kHz, whichcannot be ignored in a modulated carrier having a modulation rate of 32kbps in widely spread ADPCM voice communication digital satellitecommunication. The tolerance transmission frequency error as the errorof the transmission frequency allowed in this communication system isabout ±0.1 ppm or less.

The precision of the transmission frequency of a modulated wave emittedby an aperture terminal depends on a transmission frequency converter(to be referred as a U/C (Up Converter) hereinafter) in the transmittingsection. The U/C has a local oscillator, which determines the centertransmission frequency of the modulated carrier.

The local oscillator is formed from a synthesizer type PLL circuit. Acrystal oscillator having a frequency of several ten MHz (e.g., 10 MHz)is used as an oscillation source. This frequency is multiplied and usedby the local oscillator (e.g., for a 10-MHz oscillation source, amultiple of 1,400 yields 14 GHz). If the oscillation source has an errorof 1 ppm, this error is also multiplied and appears as an error of 1 ppmeven in the local oscillator.

The precision of the oscillation source determines the centertransmission frequency of the modulated carrier. For this reason, thecrystal oscillator serving as an oscillation source must maintain highprecision.

In general, a temperature-compensated crystal oscillator called OCXO(Oven Controlled Crystal Oscillator) (to be also referred to as an OCXOhereinafter) is widely used as a crystal oscillator which can maintainhigh precision. The OCXO is a thermostatic crystal oscillator whichincorporates a heater for generating heat, has a crystal oscillator orcrystal oscillation circuit confined in a stable-temperature oven, andrealizes very high frequency stability. In general, the OCXO precisioncan be kept at about ±0.005 ppm to ±0.01 ppm.

When the OCXO is activated (circuit is powered on), the frequencyprecision is as low as several ten ppm until the internal oven is warmedup and serves as a thermostat to obtain a stable temperature. Thestartup time until the frequency is stabilized is several min under thepresent circumstances.

In other words, during several min until the frequency is stabilizedafter power-on operation, the OCXO cannot be used as the oscillationsource of the PLL constituting the local oscillator. If a transmissionsignal is transmitted during this period, interference with an adjacentcarrier occurs.

FIG. 2 is a graph showing an example of the oscillation frequency errorof the OCXO upon power-on operation. The abscissa represents the lapsetime after the power supply is turned on, and the ordinate representsthe error from a rated frequency. For example, if a modulated carrier isemitted in the presence of an error of 10 ppm, the center frequency ofthe modulated carrier shifts to a frequency different by 140 kHz in the14-GHz band, and the carrier interferes with an adjacent carrier.

The transmission control system disclosed in the above reference hasbeen known as a conventional technique for preventing transmission of aninvalid signal upon turning on the power supply. The radio transmitterdisclosed in Japanese Examined Patent Publication No. 60-059778 uses apower-on timer for inhibiting transmission only during a predeterminedtime simply after the power supply is turned on. This control systemstarts transmission after a predetermined time even if the oscillatorcannot reach a steady state owing to any fault and the frequency errorfrom an original frequency is large. This system cannot be applied toFDMA transmission control. The transmission output control systemdisclosed in Japanese Patent No. 2944480 uses information received froma partner receiver. However, the received information is used for onlythe reception level so as to control the transmission output level.After the power-on timer operates, a transmission signal is outputregardless of a frequency error from the center frequency of atransmission carrier. For example, even if an oscillator for determininga transmission frequency malfunctions and oscillates at an abnormalfrequency, a transmission signal is output at the abnormal frequencyafter a predetermined time. Also, this control system cannot be appliedto FDMA transmission control.

The transmission control systems disclosed in Japanese Unexamined PatentPublication Nos. 64-001335 and 09-135178 monitor an output (APC voltage)from a phase comparator. Even when the frequency of a referencefrequency oscillator suffers a large error of, e.g., about several tenppm from a specified frequency, the frequency is determined normal. Thiscontrol system undesirably transmits a carrier having a large frequencyerror.

From this, the prior arts cannot be applied to transmission control ofthe modulator of a central station when FDMA communication is performedvia a satellite repeater between one central station and a plurality ofaperture terminals or between the plurality of aperture terminals in adigital satellite communication system constructed by the centralstation, the plurality of very small aperture terminals, and onecommunication satellite.

The influence of the frequency error of a transmission signal from eachaperture terminal can be avoided by setting a wide occupied bandwidthusable in the satellite repeater. This is disadvantageous in effectiveuse of the frequency and the cost.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the conventionalsituation, and has as its object to provide a satellite communicationtransmission control system and small aperture terminal capable ofpreventing transmission of a modulated carrier in a power-on periodduring which the frequency of a local oscillator becomes unstable.

To achieve the above object, according to the first aspect of thepresent invention, there is provided a satellite communicationtransmission control system which comprises one central station, aplurality of small aperture terminals, and one communication satelliteserving as a repeater for the central station and the apertureterminals, and performs communication via a frequency division multipleaccess communication channel, wherein the central station comprises amodulator for transmitting via a common channel a signal forcontrolling/monitoring the small aperture terminals on the basis of astable clock signal, and each of the aperture terminals comprises amodulator for modulating communication information from an informationsource, a transmission frequency converter for frequency-converting amodulated wave from the modulator into a carrier signal (modulatedcarrier), an antenna/RF device for exchanging signals between theaperture terminal and the central station, a reception frequencyconverter for frequency-converting a modulated carrier from the centralstation, a demodulator for demodulating a signal received via the commonchannel out of a frequency-converted received signal and outputting areception clock, and a frequency comparator for detecting a frequencyerror of a local oscillator arranged in the transmission frequencyconverter by using a clock signal from the demodulator as a referenceupon power-on operation, and generating a control signal for stoppingtransmission of the modulated carrier until the frequency error fallswithin a preset tolerance range.

The small aperture terminal defined in the first aspect furthercomprises a switch arranged on an output side of the modulator, and theswitch is kept off by the control signal from the frequency comparatoruntil the frequency error falls within the preset error range.

The small aperture terminal defined in the first aspect furthercomprises a frequency conversion mixer arranged in the transmissionfrequency converter so as to multiply an output from the modulator by anoutput from the local oscillator, and a switch inserted in a signal pathof a local signal from the local oscillator to the mixer, and the switchis kept off by the control signal from the frequency comparator untilthe frequency error falls within the preset error range.

The small aperture terminal defined in the first aspect furthercomprises a modulator local oscillator arranged for the modulator, and aswitch inserted in a signal path of a local signal from the modulatorlocal oscillator to the modulator, and the switch is kept off by thecontrol signal from the frequency comparator until the frequency errorfalls within the preset error range.

The frequency comparator defined in the first aspect comprises atolerance range setting unit for setting a frequency error range, atimer for setting a predetermined time specified by the clock signal asa gate ON period, a counter for counting outputs from the localoscillator in the gate ON period, and a comparator for comparing a countvalue of the counter with a preset tolerance range.

The local oscillator defined in the first aspect includes a synthesizerusing a temperature-compensated crystal oscillator as an oscillationsource, and outputs a multiplied output of the temperature-compensatedcrystal oscillator.

As is apparent from the above aspects, according to the presentinvention, transmission of an unstable modulated carrier caused byvariations in the frequency of the local oscillator immediately afterpower-on operation can be prevented by the small aperture terminal onthe basis of a stable clock signal obtained from a signal received fromthe central station via the control/monitoring common channel. Theoccupied bandwidth of the satellite repeater for outputting a modulatedcarrier need not be widened, and the frequency band can be effectivelyused. Interference with an adjacent modulated carrier can also beprevented.

The present invention executes absolute comparison between an outputfrom the local oscillator and a stable reference clock, and controlstransmission of a modulated carrier. The modulated carrier istransmitted immediately when the frequency error falls within apredetermined frequency error range. Compared to a method oftransmitting a modulated carrier a predetermined time after the timeroperates upon power-on operation, the present invention achieves thefollowing prominent effects. That is, transmission of a modulatedcarrier can be prevented without failing to detect oscillation at anabnormal frequency due to a fault in an oscillator for determining anoutput frequency. In addition, transmission can be performed after theoutput frequency recovers to a normal one.

The above and many other objects, features and advantages of the presentinvention will become manifest to those skilled in the art upon makingreference to the following detailed description and accompanyingdrawings in which preferred embodiments incorporating the principle ofthe present invention are shown by way of illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views showing examples of the frequency allocationof a modulated carrier in the FDMA system;

FIG. 2 is a graph showing an example of the oscillation frequency errorof an OCXO upon power-on operation;

FIGS. 3A and 3B are conceptual views showing the overall arrangement andcommunication function of a digital satellite communication systemaccording to the present invention that is constituted by one HUBstation, a plurality of VSAT stations, and one communication satellite;

FIG. 4 is a block diagram showing an arrangement of the VSAT station ina satellite communication transmission control system of the presentinvention;

FIG. 5 is a block diagram showing an arrangement of the HUB station inthe satellite communication transmission control system of the presentinvention;

FIG. 6 is a block diagram showing a detailed arrangement of a localoscillator 32 in the VSAT station;

FIG. 7 is a block diagram showing a detailed arrangement of a frequencycomparator 7 in the VSAT station;

FIG. 8 is a block diagram showing another arrangement of the VSATstation in the satellite communication transmission control system ofthe present invention; and

FIG. 9 is a block diagram showing still another arrangement of the VSATstation in the satellite communication transmission control system ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The satellite communication transmission control system for a digitalsatellite communication system constituted by one central station (to bereferred to as a HUB station hereinafter), a plurality of very smallaperture terminals (VSAT; to be referred to as VSAT stationshereinafter), and a communication satellite (to be referred to as a SAT)serving as repeater for the HUB and VSAT stations will be described indetail as a preferred embodiment of the present invention with referenceto the accompanying drawings.

FIGS. 3A and 3B are conceptual views showing the overall arrangement andcommunication function of a digital satellite communication systemaccording to the present invention using one HUB station, a plurality ofVSAT stations, and one SAT. As shown in FIG. 3A, the digital satellitecommunication system is constituted by a SAT having a repeater, a HUBstation (HUB) connected to a central computer and information terminal,and a plurality of VSAT stations (VSAT-A, VSAT-B, . . . ) each connectedto a personal computer and an information terminal such as a telephone.FDMA (Frequency Division Multiple Access) communication is performed viathe satellite repeater between the HUB station and a plurality of VSATstations or between a plurality of VSAT stations.

This digital satellite communication system uses a common signalingchannel (to be referred to as a CSC) serving as a common channel forcontrolling/monitoring via the satellite repeater all VSAT stationswhich participate in satellite communication from the HUB station, anddata communication channels between the VSAT stations.

The common channel (CSC channel) is a channel for performingcommunication by using a single frequency between a plurality ofstations by time division. This channel executes communication using theTDM (Time Division Multiplexing) method, TDMA (Time Division MultipleAccess) method, or slotted ALOHA method.

As shown in FIG. 3B, the common channel (CSC channel) is made up of anOB (Out Bound) from the HUB station to the VSAT station and an IB (InBound) from the VSAT station to the HUB station. Time division controlsignals having a UW (Unique Word) frame structure are exchanged via OBand IB using different frequency bands.

The VSAT stations are controlled/monitored by the HUB station via theCSC channel. The VSAT stations exchange modulated carriers between themand perform FDMA communication by using channels of different datacommunication frequency bands at frequencies different from that of theCSC channel. The communication control procedures are as follows.

As shown in FIG. 3A, the HUB station always uses the OB of the CSCchannel to monitor/control the VSAT stations by health check control ofeach VSAT station, channel assignment control upon reception of arequest from the VSAT station, and the like (see arrows {circle around(1)}-A and {circle around (1)}-B). For example, when a request forcommunication with the VSAT station VSAT-B occurs in the VSAT stationVSAT-A, the VSAT station VSAT-A requests assignment of a datacommunication channel of the HUB station via the IB of the CSC channel(see an arrow {circle around (2)}). Then, the HUB station assigns datacommunication channels to the VSAT stations VSAT-A and VSAT-B (seearrows {circle around (1)}-A and {circle around (1)}-B). The VSATstations VSAT-A and VSAT-B bidirectionally communicate with each otherby using the different data communication channels assigned by the HUBstation (see an arrow {circle around (3)})

To reliably, stably execute this communication control, the HUB stationcomprises an oscillator with very high frequency precision thatgenerates a control signal transmission clock to be transmitted to theCSC channel. The HUB station performs very stable operation without anypower failure.

FIG. 4 is a block diagram showing the first arrangement example of theVSAT station used in the satellite communication transmission controlsystem of the embodiment. As shown in FIG. 4, the VSAT station comprisesa modulator 1 for modulating communication information from aninformation source, a switch 2 for enabling/disabling a modulated wavefrom the modulator 1, a transmission frequency converter 3 forfrequency-converting the modulated wave from the switch 2 into an FDMAcarrier signal (modulated carrier), an antenna/RF device 4 made up of anRF circuit and a parabolic antenna for exchanging signals with the HUBstation, a reception frequency converter 5 for frequency-converting amodulated carrier from the HUB station, a demodulator 6 for demodulatinga CSC channel signal out of the frequency-converted received signal andoutputting a reception clock, and a frequency comparator 7.

The modulator 1 modulates communication information from the VSATstation and outputs a modulated wave. The switch 2 is turned on/off by acontrol signal from the frequency comparator 7, and controls input ofthe modulated wave to the transmission frequency converter 3.

The transmission frequency converter 3 functions as an up converter forincreasing the frequency of the modulated wave input via the switch 2 toa carrier frequency to be emitted by the VSAT station. The transmissionfrequency converter 3 incorporates a local oscillator 32.

The local oscillator 32 is formed from a synthesizer type PLL circuitincluding an OCXO serving as an oscillation source (reference frequencyoscillator) using a crystal oscillator. The local oscillator 32 outputsa local signal prepared by multiplying the output frequency of the OCXO(see FIG. 6 for details).

The reception frequency converter 5 has the function of a down converterfor decreasing the frequency of a modulated carrier from the antenna/RFdevice 4 to the operation frequency of the demodulator 6. Thedemodulator 6 demodulates a CSC channel signal and outputs a receptionclock.

The frequency comparator 7 compares the reception clock demodulated fromthe CSC channel by the demodulator 6 with the frequency of an outputfrom the local oscillator 32. If the error falls within a predeterminedfrequency error range, the switch 2 is turned on to pass a modulatedwave; otherwise, the switch 2 is turned off, and a control signal forinhibiting the modulated wave from passing is output.

As a modified embodiment, instead of the output from the localoscillator 32, it can be possible to use a sole output from the OCXO 321itself, shown in FIG. 6.

FIG. 5 is a block diagram showing an arrangement of the HUB station usedin the satellite communication transmission control system of theembodiment. As shown in FIG. 5, the HUB station is constituted by amodulator 9, a highly stable oscillator 8 for generating a transmissionclock, a transmission frequency converter 10, and an antenna/RF device11.

The modulator 9 is a transmission modulator for a CSC channel controlsignal or the like that controls/monitors each VSAT station. The controlsignal or the like is generated based on the transmission clock of avery-high-precision frequency from the oscillator 8. The transmissionfrequency converter 10 has the function of an up converter forincreasing the frequency of the modulated wave such as the controlsignal to the carrier frequency of a modulated carrier to be emitted bythe HUB station. The antenna/RF device 11 has a transmission/receptionfunction with respect to each VSAT station.

FIG. 6 is a block diagram showing a detailed arrangement of the localoscillator 32 in the VSAT station. The local oscillator 32 has a PLLcircuit arrangement made up of an OCXO 321, a 1/m frequency divider (m:positive integer) 322 for frequency-dividing an output from the OCXO321, a voltage-controlled oscillator (VCO) 323, a 1/n frequency divider(n: positive integer) 324 for frequency-dividing an output from thevoltage-controlled oscillator 323, a phase comparator 325 for comparingthe phases of outputs from these frequency dividers, a low-pass filterfor extracting the low frequency component of a phase error signal fromthe phase comparator 325, and a loop amplifier 327 for amplifying thelow frequency signal.

FIG. 7 is a block diagram showing a detailed arrangement of thefrequency comparator 7 in the VSAT station. The frequency comparator 7is comprised of a tolerance range setting unit 71, comparator 72, timer73, gate 74, and counter 75. The tolerance range setting unit 71 is asetting unit for setting the range of frequency errors. The timer 73turns on the gate 74 during a predetermined time specified by areception clock. The counter 75 counts outputs from the local oscillator32 during the predetermined time. The comparator 72 outputs the controlsignal in accordance with whether the count value of the counter 75falls within the range of values set in advance in the tolerance rangesetting unit 71.

Startup operation when the station is powered on in the satellitecommunication transmission control system of the embodiment will beexplained.

The VSAT station shown in FIG. 4 is OFF. Communication of the digitalsatellite communication system via the satellite repeater is in a coldstandby state. The switch 2 of the VSAT station is OFF. In this state,the apparatus of the VSAT station is powered on, and then the switch 2is turned on from the OFF state in response to the power-on operation.The local oscillator 32 constituted by the PLL circuit having the OCXO321 shown in FIG. 6 as a reference frequency oscillator is also poweredon, and the VSAT station is activated.

In the PLL circuit, the voltage-controlled oscillator 323 is controlledby an error signal output from a low-pass filter 326 via the loopamplifier 327 so as to adjust the phase difference between an outputsignal prepared by frequency-dividing an output from the OCXO 321 by 1/mand an output signal from the voltage-controlled oscillator 323 to apredetermined phase difference determined by the characteristic of thephase comparator 325, typically a phase difference of 0. With thisprinciple, the PLL circuit outputs as a local signal a multiplied output(fvco=n/m·fref) obtained by multiplying the frequency (fvco) of anoscillation output from the voltage-controlled oscillator 323 by n/m(n/m>>1) of the reference oscillation frequency (fref) of the OCXO 321.

The output frequency of the OCXO 321 exhibits low oscillation frequencyprecision immediately after power-on operation. Stabilization of theoscillation frequency to a specified value requires a predeterminedtime. Especially in the OCXO, the frequency precision is unstable untilthe internal oven is warmed up and serves as a thermostat to obtain astable temperature after the power supply is turned on. Thus, thefrequency error is large, and the error of the center frequency fordetermining a transmission frequency is also large in the localoscillator formed from the PLL circuit which uses the OCXO as areference frequency oscillator. For example, the frequency precision ofthe OCXO upon power-on operation may decrease to about several ten ppm.The frequency (fvco) of an oscillation output from thevoltage-controlled oscillator 323 during this period becomes(n/m)×several ten ppm. Hence, the degree of the frequency errorincreases.

The demodulator 6 in the VSAT station is generally so set as to receivea CSC channel sent from the HUB station. CSC channel reception operationstarts immediately after the power supply is turned on. As describedabove, the CSC channel clock is specified by an oscillator having veryhigh frequency precision. A highly stable control signal is transmitted,and the reception clock at which the control signal is demodulated alsohas a high-precision frequency.

The frequency comparator 7 compares the frequency of a local signal fromthe local oscillator 32 with reference to the high-precision receptionclock (reference clock) from the demodulator 6. The frequency comparator7 detects the frequency error, and outputs a control signal to theswitch 2. More specifically, if the frequency error falls within apredetermined range, the frequency comparator 7 turns on the switch 2;otherwise, outputs a control signal for turning off the switch 2.

As described above, the frequency error of the OCXO 321 is largeimmediately after power-on operation, and the frequency error of amultiplied local signal is large. Therefore, the comparison result ofthe frequency comparator 7 falls outside the error range. The switch 2in the OFF state in power-on operation is controlled to maintain the OFFstate on the basis of the comparison result of the frequency comparator7. A modulated wave from the modulator 1 to the transmission frequencyconverter 3 is disabled.

Thereafter, if the frequency error of the local signal decreases and thefrequency error of the local signal falls within the error range, thecomparison result of the frequency comparator 7 is switched to turn onthe switch 2. Then, a modulated wave from the modulator 1 is input tothe transmission frequency converter 3. The transmission frequencyconverter 3 converts the modulated wave into an intermediate frequencysignal or radio frequency signal. Finally, a modulated carrier having ahigh-precision center frequency is output to the antenna/RF device 4,and transmitted from the antenna.

This arrangement and operation prevent transmission of an unstablemodulated carrier in activation even if the VSAT station is activated bypower-on operation and participates in communication at discretion. VSATstations can continue and start normal communication via a TDMAcommunication channel without any interference between them.

The operation of the frequency comparator 7 will be explained withreference to FIG. 7. The frequency comparator in FIG. 7 is based on theprinciple of counting local signals during a predetermined timedetermined by the repetitive frequency of a reception clock anddetermining the deviation between frequencies. More specifically, thetimer 73 outputs a pulse signal of a predetermined time by using ahigh-frequency-precision reception clock as a reference. The gate 74 isturned on only during the pulse period, and the counter 75 counts localsignals from the local oscillator 32 in the pulse period. The comparator72 checks whether the count value of the counter 75 during the periodfalls within the range of set values such as upper and lower limits setin advance in the tolerance range setting unit 71. Then, the comparator72 outputs a control signal. An example of numerical values is asfollows.

For example, when the predetermined time of the pulse signal is 1 sec,the timer 73 counts 35,000 reception clocks for a 35-kHz receptionclock, or 12,800 reception clocks for a 128-kHz reception clock. Thetimer 73 can output a 1-sec pulse signal.

During 1 sec, the gate 74 is turned on, and outputs from the localoscillator as signals to be measured are counted by the counter. If thenominal frequency of the OCXO is 10 MHz, the count value of the countermust be 10,000,000 after 1 sec. If the count value is 10,000,001, theerror is 1×10⁻⁷.

The measured frequency error is compared with the tolerance range set inadvance in the tolerance range setting unit 71. Finally, an output fromthe frequency comparator is generated. For example, the comparator 72compares the measured frequency error with a frequency error range of,e.g., ±α×10⁻⁷ set in the tolerance range setting unit 71. The comparator72 checks whether the measured frequency error falls within the setrange, and outputs the above-mentioned control signal as a frequencycomparator output. This frequency error detection operation is repeatedafter power-on operation. If the frequency of the local oscillatorgradually stabilizes and falls within this range, the switch 2 is turnedon to start predetermined transmission operation.

A binary counter (ripple counter or the like) is used as the timer 73,and is so constituted as to generate and output a frequency-dividedoutput having 50% the duty of the reception clock from a predeterminedcounter stage. The timer 73 can output a pulse signal which enablesrepeating the frequency error detection operation. The time of the timerserving as a reference can be appropriately changed. Prolonging thistime yields high frequency error measurement precision.

In the above-described embodiment, the switch is arranged on the outputside of the modulator as a means for preventing transmission of anunstable modulated carrier. However, this means can be variouslymodified. Instead of arranging the switch on the output side of themodulator, a means for substantially stopping output of a modulatedcarrier to the antenna/RF device 4 may be disposed.

FIGS. 8 and 9 are block diagrams showing other arrangement examples ofthe VSAT station used in the satellite communication transmissioncontrol system of the present invention. FIGS. 8 and 9 show otherconcrete means for stopping transmission of a modulated carrier. In thesecond arrangement example shown in FIG. 8, the transmission frequencyconverter 3 incorporates a frequency conversion mixer 31 for multiplyingan output from the local oscillator 32 and a modulated wave from themodulator 1, as the means for stopping a modulated carrier from thetransmission frequency converter 3 to the antenna/RF device 4. Inaddition, a switch 12 is inserted in the signal path of a local signalfrom the local oscillator 32 to the frequency conversion mixer 31. Thelocal signal is ON/OFF-controlled by an output from the frequencycomparator 7. In the third arrangement example shown in FIG. 9, a switch14 is arranged on the output side of a modulator local oscillator 13which is generally disposed in the modulator 1 for modulating basebandcommunication information. The switch 14 is so constituted as toON/OFF-control a local oscillation signal having an intermediatefrequency by an output from the frequency comparator 7.

As the means for stopping transmission of a modulated carrier, a switchfor ON/OFF-controlling a modulated carrier can be interposed between thetransmission frequency converter 3 and the antenna/RF device 4. Also, atleast two of these switches can be combined. In short, the means forstopping transmission of a modulated carrier can be implemented by afunction capable of stopping the modulated carrier itself.

A wide error range may be set for a large tolerance frequency error ofthe center frequency in the FDMA method in relation to the modulatedcarrier stop period. The tolerance range set in the frequency comparatorcan be freely set. With this arrangement, the time until the modulatedcarrier can be output can be changed in accordance the tolerancefrequency error of the center frequency in the FDMA method.

1. A satellite communication transmission control system which comprisesone central station, a plurality of small aperture terminals, and onecommunication satellite serving as a repeater for the central stationand the aperture terminals, and performs communication via a frequencydivision multiple access communication channel, wherein the centralstation comprises a modulator for transmitting via a common channel asignal for controlling/monitoring the small aperture terminals on thebasis of a stable clock signal, and each of the aperture terminalscomprises a modulator for modulating communication information from aninformation source, a transmission frequency converter forfrequency-converting a modulated wave from the modulator into a carriersignal (modulated carrier), an antenna/RF device for exchanging signalsbetween the aperture terminal and the central station, a receptionfrequency converter for frequency-converting a modulated carrier fromthe central station, a demodulator for demodulating a signal receivedvia the common channel out of a frequency-converted received signal andoutputting a reception clock, and a frequency comparator for detecting afrequency error of a local oscillator arranged in the transmissionfrequency converter by using a clock signal from the demodulator as areference upon power-on operation, and generating a control signal forstopping transmission of the modulated carrier until the frequency errorfalls within a preset tolerance range.
 2. A system according to claim 1,wherein the small aperture terminal further comprises a switch arrangedon an output side of the modulator, and the switch is kept off by thecontrol signal from the frequency comparator until the frequency errorfalls within the preset error range.
 3. A system according to claim 1,wherein the small aperture terminal further comprises a frequencyconversion mixer arranged in the transmission frequency converter so asto multiply an output from the modulator by an output from the localoscillator, and a switch inserted in a signal path of a local signalfrom the local oscillator to the mixer, and the switch is kept off bythe control signal from the frequency comparator until the frequencyerror falls within the preset error range.
 4. A system according toclaim 1, wherein the small aperture terminal further comprises amodulator local oscillator arranged for the modulator, and a switchinserted in a signal path of a local signal from the modulator localoscillator to the modulator, and the switch is kept off by the controlsignal from the frequency comparator until the frequency error fallswithin the preset error range.
 5. A system according to claim 1, whereinthe frequency comparator comprises a tolerance range setting unit forsetting a frequency error range, a timer for setting a predeterminedtime specified by the clock signal as a gate ON period, a counter forcounting outputs from the local oscillator in the gate ON period, and acomparator for comparing a count value of the counter with a presettolerance range.
 6. A system according to claim 1, wherein the localoscillator includes a synthesizer using a temperature-compensatedcrystal oscillator as an oscillation source, and outputs a multipliedoutput of the temperature-compensated crystal oscillator.
 7. A smallaperture terminal in a satellite communication transmission controlsystem that is controlled/monitored by a signal based on a stable clocksignal transmitted from one central station to a common channel of asatellite repeater and performs communication via a frequency divisionmultiple access communication channel, comprising a modulator formodulating communication information from an information source, atransmission frequency converter for frequency-converting a modulatedwave from said modulator into a carrier signal (modulated carrier), anantenna/RF device for exchanging signals between said aperture terminaland the central station, a reception frequency converter forfrequency-converting a modulated carrier from the central station, ademodulator for demodulating a signal received via the common channelout of a frequency-converted received signal and outputting a receptionclock, and a frequency comparator for detecting a frequency error of alocal oscillator arranged in said transmission frequency converter byusing a clock signal from said demodulator as a reference upon power-onoperation, and generating a control signal for stopping transmission ofthe modulated carrier until the frequency error falls within a presettolerance range.
 8. A station according to claim 7, wherein said smallaperture terminal further comprises a switch arranged on an output sideof said modulator, and said switch is kept off by the control signalfrom said frequency comparator until the frequency error falls withinthe preset error range.
 9. A station according to claim 7, wherein saidsmall aperture terminal further comprises a frequency conversion mixerarranged in said transmission frequency converter so as to multiply anoutput from said modulator by an output from said local oscillator, anda switch inserted in a signal path of a local signal from said localoscillator to said mixer, and said switch is kept off by the controlsignal from said frequency comparator until the frequency error fallswithin the preset error range.
 10. A station according to claim 7,wherein said small aperture terminal further comprises a modulator localoscillator arranged for said modulator, and a switch inserted in asignal path of a local signal from said modulator local oscillator tosaid modulator, and said switch is kept off by the control signal fromsaid frequency comparator until the frequency error falls within thepreset error range.
 11. A station according to claim 7, wherein saidfrequency comparator comprises a tolerance range setting unit forsetting a frequency error range, a timer for setting a predeterminedtime specified by the clock signal as a gate ON period, a counter forcounting outputs from said local oscillator in the gate ON period, and acomparator for comparing a count value of said counter with a presettolerance range.
 12. A station according to claim 7, wherein said localoscillator includes a synthesizer using a temperature-compensatedcrystal oscillator as an oscillation source, and outputs a multipliedoutput of said temperature-compensated crystal oscillator.
 13. Asatellite communication transmission control system which comprises onecentral station, a plurality of small aperture terminals, and onecommunication satellite serving as a repeater for the central stationand the aperture terminals, and performs communication via a frequencydivision multiple access communication channel, wherein the centralstation comprises a modulator for transmitting via a common channel asignal for controlling/monitoring the small aperture terminals on thebasis of a stable clock signal, and each of the aperture terminalscomprises a modulator for modulating communication information from aninformation source, a transmission frequency converter forfrequency-converting a modulated wave from the modulator into a carriersignal (modulated carrier), an antenna/RE device for exchanging signalsbetween the aperture terminal and the central station, a receptionfrequency converter for frequency-converting a modulated carrier fromthe central station, a demodulator for demodulating a signal receivedvia the common channel out of a frequency-converted received signal andoutputting a reception clock, and a frequency comparator for detecting afrequency error of an oven controlled crystal oscillator (OCXO) arrangedin the transmission frequency converter by using a clock signal from thedemodulator as a reference upon power-on operation, and generating acontrol signal for stopping transmission of the modulated carrier untilthe frequency error falls within a preset tolerance range.